The present invention relates to a multilevel optical signal transmitter.
A conventional optical communications system uses a multilevel optical signal generated by a conventional transmitter which comprises a laser diode or a light emitting diode (LED) which is driven by a multilevel driving current. Thus, a transmitter of this type transmits a multilevel optical signal having optical intensity levels. The intensity level of the optical signal is approximately proportional to the driving current. The difference between the intensity levels of adjacent ones of the optical signals will be hereinafter called a "level-to-level distance".
In the transmitter, it is difficult to make level-to-level distances equal to each other, because it is hard to precisely control the current level of the driving current and because the characteristics of the diode which is used fluctuates due to external factors, such as temperature. Consequently, it is not easy to construct a reliable optical communications system by merely resorting to the conventional transmitter.
An object of the invention is therefore, to provide a multilevel optical transmitter which is free from the above-mentioned disadvantages of the conventional transmitter.